1. Field of the Invention
The present invention generally relates to electrostatic imaging systems and, more particularly, is concerned with an electrostatic system for multicolor imaging using microcapsular photosensitive toner particles deposited on an imagewise charged transfer surface.
2. Description of the Prior Art
One prior art imaging system disclosed in U.S. Pat. Nos. 4,399,209 and 4,440,846 and U.K. Pat. No. 2,113,860 assigned to Mead Corporation employs a paper or film web which is uniformly coated with photosensitive, chromogenic microcapsules. First, the uniformly coated web is imagewise exposed to actinic radiation and, then, the capsules are ruptured by pressure to contact their chromogenic contents with a developer, usually carried on a separate support, to produce a color image. A major disadvantage of this approach is that a precoated web of paper or film must be provided. This requirement creates problems in storage and handling of the precoated web and susceptibility of its capsules to breakage which results in unwanted background in the image.
Another prior art imaging system disclosed in U.S. Pat. No. 4,801,949 assigned to Seiko Instruments Inc. employs a container which separately stores the microcapsules prior to use and permits use of plain paper. The microcapsules are electrostatically drawn from the container and coated evenly or uniformly on the surface of an imaging body, such as on a photoreceptive drum or belt. Thereafter, a desired image is optically produced on the uniform layer of microcapsules by exposure to light in the pattern of the image on the evenly coated surface of the imaging body. However, as in the above-described prior art imaging system employing an uniformly precoated web, uniform electrostatic coating of the imaging body wastes a substantial amount of the chromogenic particles, especially when reproducing text material interspersed with some inset areas of color where the coverage of the image on the page may amount to no more than 10 to 20 percent of the page. This results in a high cost per page.
Consequently, a need still exists for an imaging system which will avoid the drawbacks of the above-described prior art system.